Hi there @CandleShack,
Welcome to the Community! Thanks for asking your question regarding segments here and hope I can help.
To begin collecting first purchase dates from new customers, you'll need to set up a Flow to apply this property. The flow setup will look like this:
Trigger: Placed Order
Conditional Split: What someone has done > placed order > equals > 1 > over all time
Update Profile Property: Create Profile Property > First purchase Date > Select date data type > check Use current date when this step is reached
There is unfortunately no way to retroactively update every person's first purchase date, but there is a workaround if you were able to export your customer's first order history from within your ecommerce backend. This would allow you to apply this first purchase date as a custom profile property.
Hope this helped,
Alex